Extrusion process and apparatus



June 20, 1967 Filed Aug. 14, 1965 FIGi 5 ,G. B. DUNNINGTON ETAL 3,325,865

EXTRUSION PROCESS AND APPARATUS 2 Sheets-Sheet 1 ATTORNEY June zo, 1967 Filed Aug. 14, 1963 G. B. DUNNINGTON ETAL.

EXTRUSION PROCESS AND APPARATUS 2 Sheets-Sheet 2 FIG@ l REUBEN T. FIE GORDON B. DUNNING f. #may ATTORNEY INVENTOR United States Patent 3,325,865 EXTRUSION PRCESS AND APPARATUS Gordon Beale Dunnington and Reuben Thomas Fields,

Wilmington, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington Dei., a corporation of Delaware Filed Aug. 14, 1963, Ser. No. 302,037 12 Claims. (Cl. 18-12) This invention relates to an extrusion process and apparatus. Specifically, this invention relates to extrusion of resin material in which finely divided resin particles are continuously compacted to form a continuous essentially solid state extrudate which extrudate is then melted chiefly by the application of mechanical energy and the melted product extruded in the desired form. This invention is particularly applicable to resins which have high melt viscosity and have low surface friction against metal when in the melted or semi-melted state, and to resins which have a narrow softening temperature range and low viscosity when melted.

Resin molded powders having high melt viscosity or sharp melting points are difficult to mold and extrude. In the past such materials have been processed through con- Ventional extrusion machines, but the results have not been predictable or satisfactory. Non-uniform flow from the outlet of the extrusion machine caused by an excessive adhesion of the screw and insufficient adhesion to the extrusion barrel wall, has been a major problem in trying to extrude polymers of this type. In extruding polymer having a narrow softening temperature range and low melt viscosity, in a conventional machine, the mixture of melted and unmelted polymer is of varying viscosity, and varying viscosity affects the polymer gripping ability of the screw and results in non-uniform flow. This nonunif'orm flow somtimes called surging, has been alleviated to some extent by operating the extrusion machine at low through-put, but this is not economical.

It is an object of this invention to provide a process for the extrusion of polymers having high melt viscosity or sharp melting point, at substantially uniform flow, and

at high through-put. It is a further object to provide a new extrusion machine that can be used to extrude polymers of high melt viscosity or sharp melting point at substantially uniform flow and at high through-put. It is a further object of this invention to provide an extrusion machine in which the polymer melting section can be controlled independently of the polymer compaction section. It is a further object to provide an apparatus that can, by its uniform forward feeding action, effectively manipulate solid polymer to push molten polymer out an extrusion port. Another object of this invention is to provide a new extrusion machine that will provide better temperature control, use less energy, and Ioperate at hi-gher through-puts when operating on conventional polymers. Other objects and advantages will be apparent to one skilled in the art from the description when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a sectional partly elevational view of the metering (rear) portion of the extrusion machine.

FIGURE l-A is a sectional partly elevational view of the melting (front) portion of the extrusion machine.

FIGURE 2 is an elevational view taken along line 2-2 of FIGURE l, showing the inlet port.

FIGURE 3 is a cross-sectional view taken along line 3-3 of FIGURE 2, showing the double lead `of the screw thread and the threaded barrel.

FIGURE 4 is a cross-sectional view taken along line 4 4 of FIGURE l-A, showing the cam-like portions of the melting section and the bearing-like portions of the melting section.

FIGURE 5 shows a longitudinal sectional view of the thread of the screw.

As may be seen from the drawings, particularly FIG- URES 1 and l-A, the apparatus comprises a housing 1, having a threaded bore 2 (FIGURE l) and a smooth bore 3 (FIGURE l-A). The two sections are machined so that alignment bushing 4 and the end of melting shaft 5 fit into recess 6. The threaded bore 2 is tapered throughout part of its length. The housing 1 has an inlet port 7 located adjacent the end of the housing having the largest bore diameter, and an outlet port 8 located adjacent the opposite en-d. A double lead screw 9, located within the threaded bore 2, has the same degree and directionof taper as does the threaded bore 2. The double lead screw is suitably journaled for rotation at 10. The double lead screw has a centrally located channel 11 containing tube 12 for circulation of a heat exchange fluid. Electrical resistance heaters 18 and 19 surround portions of the housing. The double lead screw is rotated by a motor (not shown) attached to the extended portion 20' of screw 9.

The melting shaft 5, journaled for rotation at 21 by a motor (not shown) is attached to the extended portion 22 of shaft S. (This journal bearing also serves .as a seal against passage of melted polymer along the shaft.) In the embodiment illustrated, there .are three distinct sections to the melting shaft 5. Section 23 has parallel longit-udinal flutes 24 on its surface. lSection 25 forms the major portion of the melting shaft. This section contains cam-like surfaces 26 and bearing-like surfaces 27. (Surfaces 27 partially block the longitudinal iiow of melt and force the polymer to pass over the close clearance points, space 31, of the cam-like surfaces.) The cam-like surfaces are not in the same plane (see FIGURE 4 in which a cross-section shows one cam-like surface at ninety degrees to the other. Note that the horizontal cam-like surface is shown in phantom because of interposed bearing-like surface 27.) Bearing-like surfaces 27 have a recess 28 which are not aligned. As can be seen in the drawing, the cam-like surfaces are progressively slightly larger as the outlet port is approached, that is, the space 31 between the point of the cam 30 and the smooth bore 3 becomes progressively smaller as the outlet port is approached.

The third section of the mixing shaft is the extrusion head portion 32 which is longitudinally uted and circumferentially cross-tinted.

The apparatus illustrated in the drawing is the preferred structure of this invention; however, many changes may be made and a highly satisfactory extrusion machine still be had. For example, in some operations it is not necessary that the mixing shaft and the tapered threaded screw operate at different rotational speeds, and therefore the mixing shaft and the screw may be joined in a single piece and driven with a single motor from one end. The tapered threaded screw may be triple or even quadruple lead. The thread need not be of the buttress type.

The melting shaft of the screw need not have three distinct sections. In fact, any configuration of the shaft that will yield a high degree of mechanical working and little or no advancing thrust to the polymer is highly satisfactory.

The relationship between the compressive screw and the housing must be such that the finely divided polymer feed material is compressed into a solid mass as it enters the non-tapered screw section. If the screw and the housing have ratios of large diameter to small diameter of about 3 to 2, most finely divided polymers will be compressed to a solid. In the embodiment shown in the drawings the screw depth is the same throughout the length of the screw, but when working with lower bulk density polymers which compact to a greater extent, the thread depth of the screw may be altered at the inlet end by grinding out a portion of the root between the ights thus allowing the screw to take up a bigger mass of polymer. Compaction of the polymer can also be accomplished by making the thread depth of both the screw and the housing progressively shallower, instead of, or in addition to, tapering the diameter. Since the compacted polymer is moved as a solid mass at least from the point at which the screw becomes uniform in diameter, the lead angles of threads of the screw and the housing must be such that the polymer mass slips both with respect to the screw and the housing without causing internal deformation, particularly shear, of the polymer'mass. Normally the minimum lead angle of the thread of the screw is slightly greater than the lead angle of the thread of the housing, due to the difference in the diameter of the two surfaces. The minimum lead angles for the screw should be between about 15 and about 20 degrees, and the corresponding lead angle for the thread of the housing should be between about 13 to 18 degrees. In the tapered section of the screw and the housing the lead angle will, of course, vary with the degree of taper, but in general, the lead angle should be such that the distance between adjacent threads is constant throughout the length of the screw and of the housing; in other words, if the distance between flights of the screw in the non-tapered section of the screw is 1.5 inches, then the distance between flights in the tapered section of the screw should be 1.5 inches. The use of triple or quadruple lead screws and housings normally allows the use of greater lead angles, and the device will operate with lead angles up to 60 degrees. Larger lead angles result in more through-put per revolution of the compaction screw, but require a higher drive torque.

In operation, nely divided preheated polymer is fed through opening 7 into housing 1. The screw section 9 is rotated and the polymer advanced toward the melting shaft. As the polymer advances the volume between the screw flights decreases due to the decrease in diameter of the screw and/ or depth of the screw flights. This decrease in volume causes the polymer particles to be pressed together to form a solid mass. The threaded barrel is heated by external heaters. As the polymer reaches the end -of the taper of the screw, its compaction is complete. It is usually desirable to overfeed the screw slightly so that the screw tries to squeeze the polymer into a volume slightly smaller than the polymer can be cornpressed. This results in some back-dow of the polymer, but assures that the polymer is fully compressed when it enters the non-tapered portion of the screw. The nontapered portion of the screw serves mainly as a metered pumping section, which generates sufficient forward thrust to push the polymer through the melting section and out through the outlet port without disturbing the feeding and compacting action of the tapered portion.

The solid compacted polymer is advanced through the non-tapered section of the housing by unscrewing from the threaded housing and the threaded non-tapered screw. The product issuing from the end of the threaded screw adjacent the melting shaft is a solid continuous but frangible plastic tube threaded on both its outside and inside. The end of this plastic tube is immediately fragmented by the parallel longitudinally fluted portion of melting shaft 5. Since the continuous plastic tube advances continuously, the fragments are pushed thro-ugh the flutes of the parallel fluted section and into contact wit-h the cam-like mixing surfaces of the shaft where the polymer is knea-ded and mixed. The mixing portion of the housing is at a temperature at or above the melting point of the polymer. Initially, the temperature is raised to this point by means of the electrical heater 19. The speed of rotation of the melting shaft is regulated so that by the time the polymer is pushed out of the non-threaded housing through outlet port 8, the polymer is in the desired molten form. Thus, the invention allows uninterrupted uniform discharge of a molten polymer by pushing the molten polymer with solid polymer.

In the following examples which illustrate the process of this invention, all parts and percentages are in parts by weight unless otherwise noted.

Example I Using an apparatus such as that illustrated in the drawing in which the threaded non-tapered screw section was of 2-inch diameter on which the double lead right-handed buttress type threads were of 3% inch spacing (l1/2 inch pitch) and 3/8 inch depth, the largest diameter of the tapered screw section being 3 inches, polytetrauoroethylene-hexauoropropylene copolymer powder producible by the process disclosed in U.S. Patent 2,946,763, issued July 26, 1960 to Bro et al., having a specic melt viscosity of l was introduced. The left-hand threads on the housing were also of 3A inch spacing and Va inch depth. The section of the screw under the inlet port was about 2 inches long, the tapered section about 5 inches long, and the non-tapered section about 8 inches long. The melting shaft was about 18 inches long. The polymer was preheated prior to its introduction into the inlet port to about 55 C. Cold water was introduced through pipe 12 and removed through channel 11 to cool the screw. Heaters 18 were set at 250 C. and heaters 19 were set at 370 C. The screw was rotated at 16 r.p.m., and the mixing shaft at 150 r.p.m. Molten resin, at a temperature of 390 C. at the rate of 85 lbs./hr., was extruded through outlet port 8.

Example II ing were set at 150 C. The screw section was rotated at,

48 r.p.m., and the mixing shaft was rotated at 180 r.p.m. Molten nylon polymer was extruded through outlet port at the rate of lbs./hr. at 285 C.

The apparatus works equally as well on polyoxymeth-` ylene resins, polyethylene resins, polypropylene resins, polyethylene terephthalate resins and polycaprolactam resins.

We claim:

1. An apparatus which comprises a housing having a rst section having a threaded bore, said first section having an linlet port adjacent one end thereof, said threaded bore having a diameter at the end which the inlet port is adjacent that is larger than the diameter of the bore at the other end, said diameter gradually diminishing from said inlet port to a point intermediate the ends of the first section and then remaining constant throughout the remainder of the rst section, a second section tandemly located in abutting relationship to the end of the first section having the smallest bore diameter, said second section having a smooth bore of about the same diameter as the diameter of the bore of the end of the rst section against which it abuts, said second section having an outlet port located adjacent the end furthest removed from the first section, a rotatably mounted threaded multiple lead screw located `within said first section, said screw being tapered throughout part of its length to about the same degree and in -the same direction as the bore of the rst section, said screw having -a thread of opposite hand to that of the bore, and a rotatably mounted mixing shaft located within the second section.

2. The apparatus of claim 1 in which the rotatably mounted threaded multiple lead screw and the rotatably mounted mixing shaft are integral.

3. The apparatus of claim 1 in which the rotatably mounted threaded multiple lead screw and the rotatably mounted mixing shaft are distinct structures, each adapted to be driven at a different speed of rotation.

4, The apparatus of claim 3 in which the threaded multiple lead screw and the mixing shaft are adapted to be driven by power applied to the shafts at the two ends of the housing.

5. The apparatus of claim 4 in which the mixing shaft includes a parallel longitudinal uted section, a series of cam-like surfaces, and a parallel longitudinal iuted ,and parallel Vertical fluted section.

6. The apparatus of claim 5 in which the cam-like surfaces are separated by bearing-like surfaces.

7. The apparatus of claim 1 in which the depth of the thread on the screw is uniform throughout the length of the screw.

8. The apparatus of claim l in which the diameter of the screw at the inlet port is in a ratio of about 3 to 2 to the smallest diameter of the screw.

9. The apparatus of claim 8 in which the distance between screw threads is uniform throughout the length of the screw.

10. The apparatus of claim 9 in which the screw is triple lead.

11. The -apparatus of claim 9 in which the screw is quadruple lead.

12. The apparatus of claim 9 in which the lead angle of the thread of the screw -is between about 15 and about 20 degrees, and the lead angle for the thread of the housing is between about 13 and about 18 degrees.

References Cited UNITED STATES PATENTS 2,496,147 1/ 1950 Brillhart 18-12 X 2,639,464 5/ 1953 Magerkurth.

2,705,131 3/1955 Ross et al.

2,783,498 5/ 1957 Richardson 18-12 2,902,923 8/ 1959 Stacey 18-12 X 3,008,184 11/1961 Fritsch 18-12 3,178,769 3/1965 Lorenian 264-176 3,248,469 4/ 1966 Kosinsky et al. 18-12 WILLIAM J. STEPHENSON, Primary Examiner. 

1. AN APPARATUS WHICH COMPRISES A HOUSING HAVING A FIRST SECTION HAVING A THREADED BORE, SAID FIRST SECTION HAVING AN INLET PORT ADJACENT ONE END THEREOF, SAID THREADED BORE HAVING A DIAMETER AT THE END WHICH THE INLET PORT IS ADJACENT THAT IS LARGER THAN THE DIAMETER OF THE BORE AT THE OTHER END, SAID DIAMETER GRADUALLY DIMINISHING FROM SAID INLET PORT TO A POINT INTERMEDIATE THE ENDS OF THE FIRST SECTION AND THEN REMAINING CONSTANT THROUGHOUT THE REMAINDER OF THE FIRST SECTION, A SECOND SECTION TANDEMLY LOCATED IN ABUTTING RELATIONSHIP TO THE END OF THE FIRST SECTION HAVING THE SMALLEST BORE DIAMETER, SAID SECOND SECTION HAVING A SMOOTH BORE OF ABOUT THE SAME DIAMETER AS THE DIAMETER OF THE BORE OF THE END OF THE FIRST SECTION AGAINST WHICH IT ABUTS, SAID SECOND SECTION HAVING AN OUTLET PORT LOCATED ADJACENT THE END FURTHEST REMOVED FROM THE FIRST SECTION, A ROTABLY MOUNTED THREADED MULTIPLE LEAD SCREW LOCATED WITHIN SAID FIRST SECTION, SAID SCREW BEING TAPERED THROUGHOUT PART OF ITS LENGTH TO ABOUT THE SAME DEGREE AND IN THE SAME DIRECTION AS THE BORE OF THE FIRST SECTION, SAID SCREW HAVING A THREAD OF OPPOSITE HAND TO THAT OF THE BORE, AND A ROTATABLY MOUNTED MIXING SHAFT LOCATED WITHIN THE SECOND SECTION. 